According to the state of the art, oil-based drilling fluids (e.g. kerosene, diesel fuel, paraffins and/or olefins) have been the primary means of providing lubricity to the drill during the hydrocarbon exploration process. A major downside in the use of such materials has been in offshore drilling and in lake systems, mainly due to their pollutant characteristics, which can cause serious environmental problems and eventually lead to complex logistical operations. These oil-based fluids cannot be disposed of at sea or in water, lake and/or underground aquifer systems due to environmental and pollution concerns and their adverse effects on aquatic life. In these cases, oil-based fluids containing naphthas or paraffins need to be transported ashore for some suitable type of disposal or recovery. In Brazil, regulations are being increasingly tightened on the adverse environmental effects caused by the use of paraffins and their mixtures in drilling fluids, especially in offshore operations such as those involved in deep water during pre-salt exploration.
Drilling fluids commonly used in the form of so-called inverted emulsions correspond to three-phase systems, referred to as: oil, water and particulate additives, which include emulsifiers or emulsifying systems, viscosifying agents, additives to prevent loss of filler, alkaline reserves, rheological modifiers, among others, which together form a formulation tailored to the performance needs. Full details of the mixtures developed for these fluids can be found widely described in the literature.
U.S. Pat. Nos. 2,222,949, 2,316,967, 2,316,968 and 2,698,833 mention primarily non-aqueous oil-based drilling fluids which are inherently pollutants. U.S. Pat. No. 2,698,833 addresses a drilling fluid that was prepared with a thermally-stable oil having less than about 5% aromatics and unsaturates by volume. However, this oil still has polluting properties.
Oil-based drilling fluids were originally made from diesel oil fractions containing aromatic constituents. In order to detoxify and reduce the environmental impact caused by the discharge of same, the so-called “non-polluting oils”, free of aromatic fractions, were initially proposed as oil phase for drilling fluids. Although some advances have been achieved in the elimination of aromatic compounds, a severe reduction of these and other low biodegradability and high toxicity substances must be achieved quickly. This applies in particular to submerged drilling activities such as those conducted in offshore wells where the exploitation of oil and gas reserves can have a major impact on marine ecosystems, particularly sensitive to the presence of toxic and not easily biodegradable substances.
In a relevant and recognized technology for solving the problems of oil-based drilling fluids the use of continuous ester-based phases has already been described. As an example, U.S. Pat. Nos. 4,374,737 and 4,481,121 disclose ester-based drilling fluids in which the esters are synthesized from non-polluting oils, including non-aromatic mineral oil fractions and vegetable oils from peanut, soybean, flax, maize, rice oils, or even animal oils such as whale oil are mentioned as non-polluting oils in equivalent performance positions. The vegetable and animal oil esters mentioned in these inventions correspond to triglycerides and natural fatty acids which are recognized as safe for the environment and, from the ecological point of view, are differentially superior to the hydrocarbon fractions even when they have been dearomatized.
In a method described in international patent application WO 2007/137709 A1, the composition of a non-aqueous phase has been developed to decrease the toxicity of the drilling fluid in accordance with international standards ASTM E 1367-92 & EPA/600/R-94/025, Section 11. The non-aqueous, or oily phase had at least one additive which, when added to the mixture, ensured the decrease in toxicity of the mixture. Specifically, for the use of low toxicity oil-based continuous phases has been described the use of olefins, wherein olefins are understood to be linear or branched non-aromatic hydrocarbons having at least one unsaturation. The olefins used are characterized by their internal or terminal unsaturations (e.g. alpha position), as well as flash points of at least 80° C. The use of such olefins has previously been described in Canadian patent CA 2192998 A, where they were part of the oil phase, and in oil/water emulsified systems. Significantly, its use was attributed in drilling fluids, where ecological compatibility is mentioned, being biodegradable under both aerobic and anaerobic conditions. Among suitable mixtures of the components described in this patent special mention is made of the olefinic alcohols, ethers and the corresponding esters obtained from carboxylic acids and/or carbonic acid esters.
The invention disclosed in U.S. Pat. No. 4,631,136 relates to compositions for vegetable oil-based drilling fluids which are non-polluting and toxic and which provide improved lubrication properties and stability for use under widely varying loading, pressure and temperature conditions. The compositions described in that patent are primarily based on vegetable oils which comprise essentially long-chain carboxylic acids having one or more unsaturations and a minimum number of 12 to 24 carbon atoms. Among the oils that have been selected are: peanut oil, rapeseed oil, soybean oil, sunflower oil, corn oil, cottonseed oil, rice bran oil, safflower oil, castor oil, palm oil and mixtures thereof. Suitable antioxidants and emulsifiers as well as viscosifiers and rheological modifiers have been included in the described compositions and which have been adapted for addition to water and/or saline solution according to the desired rheological properties for drilling.
U.S. Pat. No. 3,761,410 discloses a water-based drilling fluid with increased lubricity, the water being dispersed as small discrete non-emulsified droplets in the fluid. This composition includes water and insoluble alkyl alcohol, a lubricant additive consisting of a vegetable oil, and a pour point depressant consisting of an alcohol or glycol. This patent reviews the lubrication characteristics required for drilling fluids, which has been the subject of much research as evidenced by the number of patents in the state of the art. See, for instance, the following U.S. Pat. Nos. 2,773,030, 2,773,031, 3,014,862, 3,027,324, 3,047,493, 3,047,494, 3,048,538, 3,214,374, 3,242,160, 3,275,551, 3,340,188, 3,372,112 and 3,377,276. These patents disclose various types of additives for use in oil-based drilling fluids, oil-in-water emulsion drilling fluids and water-based drilling fluids. It has also been proposed to incorporate an emulsifier into water-based drilling fluids wherein the lubricant additive to be used is water insoluble. Most of the prior art additives function as lubricants under extreme pressure and the drilling fluid is added primarily to provide lubricity to the drill bearings. The additives described in this case decrease the friction that occurs between the drill string and the sides of the hole, thereby increasing the lubricity of the drilling mud.
U.S. Pat. No. 4,374,737 mentions a drilling fluid composition for mixing with water which comprises large amounts of diethanolamine, tall oil fatty acid and imidazoline/amide mixed with a non-polluting and biodegradable vegetable oil comprising a small amount of the composition, the latter being used to act as an emulsifier when water is added in the composition. These fluids are described as not requiring the use of additional emulsifiers to maintain the emulsion and having the desired rheological properties comparable to those obtained with the crude oil or diesel oil; however, they require large amounts of concentrate.
International patent Application No. WO 1990/006980 A1 describes inverted emulsion-based drilling fluids which are characterized by a high degree of ecological compatibility combined with good permanent performance characteristics. The use of new drilling fluid systems has a particular significance in the marine area, but is not limited to it. The drilling fluid systems described in this patent application may be also used for drilling on land, for instance: drilling for aquifer prospecting, geoscientific drilling, etc. The principle applied here has been substantially simplified by the invention of drilling fluids and the use in oil prospecting using the selection of esters based-fluids with low or no ecotoxicity.
Moreover, Patent Application No. WO 1990/006980 refers to the use of selected esters which exhibit free flow (high flowability) and which can be pumped in a temperature range of 0-5° C. The oil phase in these inverted emulsion-based drilling fluids is suitable for use in the fields of oil and gas exploration, and does not harm the environment. However, the described esters were prepared from C2-C12 monofunctional alcohols and C16 to C24 unsaturated monocarboxylic acids, the latter having one or more olefinic double bonds. The invention also exemplifies the use of said esters described in formulations containing—as dispersed aqueous phase—a saturated brine of divalent salts, such as calcium chloride, at least one emulsifier, at least one viscosifying agent, at least one additive for fluid loss and a moderate alkaline reserve in the form of lime. Similarly, U.S. Pat. No. 5,403,822 describes esters obtained from synthetic or natural monocarboxylic acids having 6 to 11 carbon atoms and mono- and/or polyfunctional alcohols such as the oil phase or oil phase component in inverted emulsions for drilling fluids. However, the use of monoesters obtained from monocarboxylic fatty acids, such as those described in the two above-mentioned patents, under the conditions found in the pre-salt (high pressure and temperature), may show instability in the formulations of the drilling fluids, mainly due to the high propensity to hydrolysis of these esters.
According to the above, it is clear that drilling activities in unconventional environments, in which the presence of high temperatures and pressures, together with drilling through geological layers sensitive to the presence of water (e.g. pre-salt), requires the development of new molecules or mixtures of molecules providing good rheological and lubricity properties, as well as ensuring good formation stability, good hydrolytic stability, reuse and, in particular, low toxicity and high biodegradability for use in marine environments.
As it will be better described below, the present invention seeks to solve the above-described problems of the prior art in a practical and efficient manner.